1. Field of the Invention
The present invention relates to a liquid crystal composition suitable for multiplex drive and a liquid crystal display device using the same.
2. Description of the Related Art
A liquid crystal display apparatus having a plurality of pixels arranged in a matrix manner is applied to a display unit of a computer terminal, an image display unit of a television receiver, and the like. Recently, a demand has arisen for a large size and high image quality of the image display unit. Therefore, an increase in number of pixels and an improvement in contrast are desired. A liquid crystal display apparatus applied to the image display unit has a simple matrix type twisted nematic liquid crystal display device (to be referred to as a matrix TN. LC. device) arranged such that a plurality of electrodes are aligned on inner surfaces of a pair of opposing substrates and opposing portions of the electrodes form a plurality of pixels aligned in a matrix manner. The matrix TN. LC. device is driven in a multiplexed manner.
In the matrix TN. LC. device, if the number of pixels is increased in order to improve the resolution and increase a display area, the number of scanning lines is naturally increased. Therefore, high multiplex drive must be performed. However, if a multiplexing degree is increased, a difference in effective voltages between an on electric field to be applied to a liquid crystal to turn on pixels and an off electric field to be applied to the liquid crystal to turn off the pixels is reduced. As a result, an operating margin of a drive voltage is reduced, the contrast is lowered, and a viewing angle characteristic is degraded.
The operating margin and the contrast of a liquid crystal display device depend on a voltage-luminance characteristic. That is, when a change in transmittivity with respect to a change in intensity of the electric field to be applied to the liquid crystals is steep, the operating margin can be increased, and the contrast can be increased. As shown in FIG. 1, the steepness of the voltage-luminance characteristic is represented by a ratio (to be referred to as .gamma. value hereinafter) between voltage V.sub.50 at which the transmittivity is 50% and threshold voltage V.sub.C. When the .gamma. value becomes closer to 1, the change in transmittivity becomes steeper. Therefore, the operating margin can be increased, and the contrast can be increased.
In addition, in the matrix TN. LC. device which is of high multiplex drive type, a multiplexing degree is increased, and one selection period is shortened. Therefore, the matrix TN. LC. device must respond at high speed.
As described above, the matrix TN. LC. device of high multiplex drive must have:
(1) a .gamma. characteristic close to 1; PA0 (2) a wide viewing angle; and PA0 (3) a high response speed. PA0 V.sub.C : the threshold voltage PA0 K.sub.11 : the splay elastic constant of the liquid crystal PA0 K.sub.33 : the bending elastic constant of the liquid crystal PA0 .DELTA..epsilon.: the dielectric anisotropy of the liquid crystal PA0 .epsilon..perp.: the dielectric constant in a direction perpendicular to a liquid crystal molecular axis PA0 .DELTA.n: the optical anisotropy of the liquid crystal PA0 d: the liquid crystal layer thickness PA0 .lambda.: the wavelength of light PA0 q=.pi./d, K=K.sub.11 +[(K.sub.33 -2K.sub.22)/4] PA0 .eta.: viscosity PA0 .epsilon..sub.0 : dielectric field intensity PA0 E: electric field intensity PA0 K.sub.22 : torsional (twist) elastic constant
The .gamma. characteristic is studied by M. Schadt et al. According to their studies, the .gamma. value representing the steepness of the voltage-luminance characteristic is given by the following equation (I) and coincides well with the characteristic of an actual device: ##EQU1## where V.sub.50 : the applied voltage when a transmittivity of 50% is obtained
According to equation (I), it is apparent that when the first, second, and third terms of equation (I) are close to 1, the .gamma. value is close to 1. Therefore, in order to improve the .gamma. value characteristic, the following conditions must be simultaneously satisfied:
(a) a ratio (to be referred to as elastic constant ratio K.sub.33 /K.sub.11 hereinafter) of bending elastic constant K.sub.33 to splay elastic constant K.sub.11 is small;
(b) a ratio (to be referred to as dielectric ratio .DELTA..epsilon./.epsilon..perp. hereinafter) of dielectric anisotropy .DELTA..epsilon. to the dielectric constant in a direction perpendicular to the liquid crystal molecular axis; and
(c) a value of product .DELTA.n.multidot.d of liquid crystal optical anisotropy .DELTA.n and liquid crystal layer thickness d is 1.1 (.mu.m) when a wavelength of incident light is 550 nm.
Dependency (to be referred to as a viewing angle characteristic hereinafter) of the contrast to an observing direction is studied by Mr. G. BAUR and reported in "The influence of Material and Device Parameters on the Optical Characteristics of Liquid Crystal Displays", Molecula Crystals and Liquid Crystals, Volume 63, Nos. 1 to 4, 1981. According to this report, the viewing angle characteristic depends on liquid crystal layer thickness d and liquid crystal optical anisotropy .DELTA.n of a liquid crystal. That is, in a liquid crystal display device having large product .DELTA.n.multidot.d (to be referred to as .DELTA.n.multidot.d hereinafter) of layer thickness d and optical anisotropy .DELTA.n, an apparent change rate of .DELTA.n.multidot.d obtained when the liquid crystal display device is viewed from its front and in an oblique direction is large, resulting in a poor viewing angle characteristic. To the contrary, a liquid crystal display device having small .notident.n.multidot.d has a good viewing angle characteristic. In addition, when liquid crystal display devices having equal .DELTA.n.multidot.d are compared, a better viewing angle characteristic is obtained with smaller optical anisotropy .DELTA.n of the liquid crystal. That is, a better viewing angle characteristic is obtained when a change in contrast with respect to a change in observing direction is small. Therefore, in order to improve the viewing angle characteristic:
(d) .DELTA.n.multidot.d must be reduced; and
(e) .DELTA.n must be reduced.
As for the response characteristic, response time t.sub.ON required for turning on the liquid crystal display device and response time t.sub.OFF required for turning off the liquid crystal display device are represented by the following logic equations (II) and (III), respectively, and coincide well with measurement values: EQU t.sub.ON =.eta./(.epsilon..sub.0 .DELTA..epsilon.E.sup.2 -Kq.sup.2) (II) EQU t.sub.OFF =2/3/Kq.sup.2 (III)
where
According to equations (II) and (III), the response speed depends on viscosity .eta. and electric field intensity E. That is, in order to increase the response speed:
(f) viscosity .eta. must be reduced; and
(g) the electric field intensity must be increased.
Therefore, in order to obtain a steep .gamma. characteristic, the conventional liquid crystal display device has been designated to satisfy condition (c) concerning .DELTA.n.multidot.d of conditions (a) to (c). That is, a value of .DELTA.n.multidot.d is set to be 1.1 (.mu.m) because the center of a wavelength range of a visual light beam is about 550 nm. However, conditions (a) and (b) are not sufficiently taken into consideration. The reasons for this are as follows. That is, in order to reduce .DELTA..epsilon./.epsilon..perp. of condition (b), a value of .DELTA..epsilon. may be reduced. In this case, however, a response speed in reduced. In addition, since a liquid crystal composition in which K.sub.33 /K.sub.11 of condition (a) is reduced has high viscosity, a smectic phase is easily generated. As a result, the response speed is reduced, and an operation temperature range is narrowed.
Optical anisotropy .DELTA.n of the conventional liquid crystal composition generally falls within the range of 0.13 to 0.16. Therefore, if the .DELTA.n.multidot.d value is set to be 1.1 (.mu.m), liquid crystal layer thickness (interelectrode gap) d falls within the range of 7.0 to 8.8 (.mu.m). In this case, since layer thickness d is large, the intensity of an electric field is reduced to reduce the response speed. In addition, a viewing angle characteristic is poor because .DELTA.n.multidot.d is large.
In order to increase the response speed, a liquid crystal composition used in the above conventional liquid crystal display device has a cyano group at its terminal end. In this liquid crystal composition, a large amount of a liquid crystal compound in which a value of dielectric anisotropy .DELTA..epsilon. is largely positive is mixed to increase the dielectric anisotropy .DELTA..epsilon. value of the liquid crystal composition. In addition, a large amount of a liquid crystal compound having a large optical anisotropy .DELTA.n value is mixed to increase an optical anisotropy .DELTA.n value of the liquid crystal composition. Furthermore, a value of liquid crystal layer thickness d of the liquid crystal display device is reduced to increase the electric field intensity, and the value of .DELTA.n.multidot.d is set to be 1.1 (.mu.m) (.lambda.=550 nm) to obtain a good .gamma. characteristic, i.e., high contrast and a large operating margin.
However, in the above conventional liquid crystal composition, the .DELTA..epsilon./.epsilon..perp. value is, e.g., 1.1 or more because the value of .epsilon..perp. is small. In addition, in order to prevent low viscosity and a smectic phase, a mixing ratio of a liquid crystal compound with which a low-viscosity liquid crystal and a stable smectic phase can be obtained is increased. Therefore, the value of elastic constant ratio K.sub.33 /K.sub.11 is, e.g., 1.2 to 1.3 or more.
As described above, since the conventional liquid crystal composition has large .DELTA..epsilon./.epsilon..perp. and K.sub.33 /K.sub.11 values, it is difficult to improve the .gamma. characteristic of the liquid crystal display device using this liquid crystal composition. In addition, since the .DELTA.n.multidot.d value is large, it is difficult to improve the viewing angle characteristic.